Because we rarely ask our muscles to work that way. Deadlifts, especially, strengthen our bodies against the most commonly encountered stresses. While bench presses are commonly recommended, I think overhead presses and bent rows are more helpful at preventing injuries.
I'm suggesting it more for the overall balance, and I'm not advocating dropping any of the other lifts. I love me my deads, don't get me wrong.
I'll grant that the vertical pull isn't commonly encountered (swimmers excepted) unless you're climbing trees to escape lions/tigers/bears, and I do a fair bit of horizontal pulling exercises as well (open a door, lift something toward you, row a boat).
Focusing on pressing without balance pulling tends toward imbalance IMO/IME.
